Process for the production of high alpha-cellulose fiber



Jan. 3, 1928.

` G. A. RICHTER v PROCESS FOR THE PRODUCTION OF HIGH ALPHA CELLULOSE FIBER Original Filed May 12, 1925 Patented Jan. 3, 1928.

UNITED STATES PATENT OFFICE.

GEORGE A. RICHTER. OF BERLIN, NEW HAMPSHIRE. ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE. A CORPORATION OF MAINE.

PROCESS FOR THE PRODUCTION OF HIGH ALPHA-CELLULOSE PEER.

Application led May 12, 1925, Serial No. 29,713. Renewed .Tune 3, 1927.

This invention relates to the utilization of the heat and the recovery of spent sodium digesting constituentfrom the spent liquor of a plant producing high alphacellulose pulpthat is. pulp containing a high per centage of alpha or resistant cellulose,- where operated simultaneously or in con-- nection with a sulphite plant employing a sodium base cooking liquor.

High alpha-cellulose pulp may be produced by an alkaline digestion of unbleached sulphite pulp.-a caustic soda. soda ash. or a mixed solution of caustic soda and soda ash being especially adapted for such digestion. The. alkaline digestion may take place in digesters at any suitable temperatures and pressures. although I have found it tobe more economical and expedient in certain cases to digest the sulphite pulp under at,- mospheric pressure in open tanks. The alkaline solution reacts with and vdissolves the ligneous and other coloring matter, together with the less-resistant celluloses contained in the sulphite pulp. and after a suflicient period of digestion yields pulp which contains at least 9i?? alpha-cellulose and has a very white color after bleaching. .In order to secure marked economy in the produc tion of high alpha-cellulose pulp, the sodium constituent present in the spent digesting liquor should be recovered. and the available heat content of such liquor utilized.

In my companion application Serial No. 26.361. tiled April 2S, 1925. there has been outlined a process for the recovery of the spent sodium constituent and the utilization of the available heat content of the spent digesting liquor. Such a process, however. requires a special recovery plant, where it is practiced in aplant wherein the usual sulphite process. in which is employed a calcium or other alkaline earth metal sulphite liquor for the digestion of cellulosie material. is practiced.

The object of this invention is to provide a process for practicing the economies set forth in the co-pending application, while combining the recovery .of sodium constituent in an alpha-cellulose plant with the recovery of sodium constituent in a sulphite plant wherein a sodium base cooking liquor is employed for the digestion of cellulosic material. thereby eliminating thevnecessity for special recovery apparatus in the alphacellulose plant. The manner in which this object is attained and certain other features of invention involved herein may best be understood when taken in connection with the accompanying drawing. which represents what may be termed a flow sheet.

Beforeproceeding to the specic features of thisdinvention. certain factors which enter into the operation of the alpha-cellulose plant (which has been fully described in the companion application) will be briefly reviewed.

Sulphite stock is obtained. as usual, by the digestion of wood chips (e. g.. spruce) in a digester under the requisite heat and pressure conditions for the necessary period of time. in a suitable sulphite liquor. and, as indicated on the drawing. the particular sulphite liquor employed in the production of the sulphite stock may be a sodium base. liquor. The digester contents are then blown and the sulphite stock is successively washed. screened and thickened by press rolls to. say. a 50% consistency. The thickened unbleached sulphite stock thus obtained is delivered to the digesting tank A, communicating b v a slow-moving worm conveyor to the bottom portion of a second digest-ing tank B. The stock is continuously fed into the tank A and is diluted to aboutl a 10P? consistency with the required amount of caustic soda or soda ash solution and water. and the stock is fed by the worm conveyor to the tank B at the same rate at which it is made up in the tank A. The stock is digested with agitation during its passage through the digesting tanks at from 180 F. to 220"l F.. under atmospheric pressure. by blowing live steam directly into the stock suspension in the tank S,- less-resistant celluloses and the ligneous and other coloring matter contained in the sulphite pulp reacting with and being dissolved b v the caustic soda solution and leavin a pulp containing a high percentage of alp a.- cellulose. Agitation of the stock during the digestion thereof during its passage through the tanks secures intimate cont-act. between the stock and the cooking liquor. thereby promoting a thorough reaction and resulting in a uniform high alpha-cellulose pulp product. The digestion may be practiced on a batch basis. butobviously a continuous cyclic process is preferable.

'lhe digested stock, together with vthe ing constituent is taken into account; but

spent digesting liquor associated therewith leaving the top portion -of the tank B and now containing a high percentage of alphacellulose, is passed through a counter-current Washer wherein the stock is washed free from substantially all the spent digestin liquor, and the Washed stock is then passed to the bleachery. The wash Water used in the Washer assumes practically the same tem` perature as the hot spent digesting liquor and the hot pulp, as they are passed into the Washer. rlhe hot Wash water (which for convenience of designation l term the alpha spent liquor) contains substantially all the sodium constituent in any form, spent or otherwise, associated with the alpha stock as lit was passed into the Washer. rlhis hot Wash Water is recirculated to the digesting tank A, and comprises in volume approximatelyT the volume of Water associated with the pulp after digestion, plus the total volume of Wash water used in the Washer, minus that portion absorbed by the Washed alpha pulp as it passes to the bleachery.- During this stage of the process, the volume of wash Water employed in Washing the. digested stock is so regulatedl that the volume of spent alpha. liquor obtained from the Washer and recirculated into the tank A is substantially equal to the volume of Water and digesting solution employed in making up the initial 10% stock suspension in the tank Af. To this recirculated alpha spent liquor is continuously added a sufficient quantity of 50% sulphite pulp, and the amount of concentrated caustic soda or soda ash solution necessary to effectively remove the impurities contained in the sulphite pulp,-thereby again producing a stock suspension of about 10%. The 10% stock suspension formed With the recirculated spent liquor is digested, and the cycle of operations as hereinbefore described in connection with, the initial 10% stock suspension, is repeated. 'llhe hot spent alpha. liquor obtained from the Washer at this point in the process contains a higher per cent of dissolved material. This spent liquor is recirculated to the digesting tank A, as before, and the process as described is continued or repeated until a. hot. syrupy spent liquor of about 10 to 15 Baum results. uch a concentrated spent liquor may profitably be evaporated, and the combustible content thereof burned, to recover the valuable sodium constituent. When this concentration of vspent liquor is reached, the flow of fresh Water to the washer is increased. As stated previously, up tothis point the volume of spent liquor delivered from the Washer has been only suiiicient toV dilute the incoming unbleached sulphite stock to a 10% consistency, or suiicient to. ensure lsuch consistency When the volume of solution containing fresh digestthere is now an excess of spent liquor. This excess is continuously Withdrawn from the spent alpha liquor obtained from the washer, and mixed with a spent liquor of about 10o Baum from the sulphite pulp Washer. The mixture of spent liquors is passed to the evaporators. The rest of the spent alpha liquor is recirculated lto the alpha digesting tank A. rTheI increase in quant-ity of wash warten-that is, the excess passed along with the spent sulphite liquor to the evaporators,-is so adjusted as to maintain an equilibrium Within the system at this particular concentration: that is, the excess spent liquor mixed with the spent sulphite'liquor contained substantially the amount of spent sodium constituent which was formed in the preceding digestion (the term sodium or sodium constituent as used in this specification and the appended claims Will, of course` be understood to mean sodium in combined form,-that is, in the form of a sodium compound).

y It will, again, be clearly understood that the spent sulphite liquor referred to as indicated on the drawing results from the digestion of Wood chips in a digester, in a sulphite digesting liquor, employing a sodium base, which may consist either of a sulphurous acid solution of sodium sulphite or a sulphurous acid solution of sodium sulphate and sodium sulphite. After the necessary period of digestion, the sulphite. digested contents are blown into the usual blow pit, and the sulphite pulp, together with its associated spent liquor, is preferably passed into a neutralizing tank wherein the spent sulphite liquor is neutralized with alkaline sodium compounds, as Will subsequently be further described. v

The neutralized pulp, together with the neutralized spent sulphite liquor, are then passed'to a counter-current Washer similar to the Washer used in connection with the high alpha-cellulose pulp, wherein the pulp is Washed substantially free from all its associated spent sulphite liquor and yields a l0o Baume liquor. The washed sulphite stock obtained from the counter-current Washer may, or may not. be the sulphite pulp which is employed in the manufacture of the' high alpha-cellulose pulp. To the liquor from the Washer, as has been indicated. is added the excess spent liquor of from 10O to l50 BaumeJ obtained from the alpha spent liquor, so that a mixture of alpha and spent A sulphite liquor of about the same degree B'aum is passed into the evaporators for concentration.

When the sulphite liquor consists of a sulphurous acid solution of sodium sulphite, the recovery of the sodium constituent is practiced as follows. The combustible content of the resulting concentrate is burned, preferably in a combined Iboiler and smeltlln ing furnace, in a reducing atmosphere. The molten inorganic eilluent from the furnace contains essentially sodium sulphide. sodium carbonate, and a small amount of sodium sulphate, and is delivered into a dissolving tank containing just enough water to form a concentrated boiling solution of the constituents of the smelt. This solution is plassed along into a storage tank, and may rnish al portion of the alkaline digesting agent required in the alpha plant tanks. This portion. preferably such that it con tains the total alkali necessary tor the alpha digestion (together with the unspent digesting constituent present in the recirculated spent liquor) minus the losses suitered in both the alpha and sulphite pulp cycles. is continuously withdrawn from the solution in the tank and, if desired, may be filtered prior to mixing with that portion of the spent alpha liquor which is recirculated for re-employment in the alpha digesting tanks. The remainder of the smelt solution is passed into a carbonating tower in contact with and in counter-current flow to carbon dioxide. The carbon dioxide used for this purpose is preferably .obtained from waste gas. such as Hue gas, but if carbon dioxide of higher concentration is desired. this may be obtained from other` suitable sources, as will presently be indicated. The sodium sulphide contained in the solution 4reacts with the carbon dioxide in the carbonating tower, forming sodium carbonate` with the evolution of hydrogen sulphide: that is. the sodium sulphide is converted into sodium carbonate with the liberation of sulphuretted hydrogen, so that the etiluent liquor from the carbonating tower consists essentially of a sodium carbonate solution. A certain small quantity of this 'sodium carbonate liquor may be used to neutralize the slightly' `through the counter-current washer.

acid spent liquor in the neutralizing tank prior to the passage of the sulphite pulp The remaining liquor is sent back to the cooking liquor making system, where it is treated with sulphur dioxide and converted into the usual sodium bisulphite liquor containing free sulphurous acid. The carbon dioxide liberated from the sodium ycarbonate solution during its treatmentwith sulphur dioxide may be recovered and employed in the carbonating tower for the carbonation of smelt. liquor as hereinbefore described. The

" hydrogen sulphide emerging from the top of the carbonating tower may also berecovered in some suitable manner. as indicated on the drawing-as by burning it with air, thereby forming sulphur dioxide which may be employed in the acid liquor making plant in the preparation of the digesting liquor from the sodium carbonate liquor.

The new causticl required for making up losses in both digesting cycles is added in the alpha digesting tank A, and, together with the portion of liquor containing recovered sodium carbonate. will vield a liquor containing suliicient digesting agent for the preparation of the high alpha-cellulose fiber. This means that if the total cycle losses in the sulphite plant are equivalent to the amount of alkali necessary to etiect digestion of the unbleached sulphite stock, no sodium carbonate liquor from the recovered liquor will be available in the alpha digesting tanks.

I have found'that the use of a small amountl of sulphide is notobjectionable in the alpha treating tanks, but if there is any reason for avoiding the use of sulphides, an alternative method of recovery may be employed as presented by the dash lines: that is. the recovered liquor passed along to the alpha digesting tanks would contain only sodium carbonate. In such a case, the digesting liquor used in converting the sulphite stock into high alpha-cellulose stock would contain sodium carbonateand caustic soda. It it is desired to make up losses in both cycles by the addition of soda ash to the alpha tanks, the digesting liquor in this case would consist only of sodium carbonate.

YVhere the sulphite liquor consists of a sulphurous acid solution of sodium sulphate and sodium sulphite, the spent liquor, which may or may not be neutralized prior to the washing of the sulphite pulp, is mixed with and neutralized prior, to the concentration thereof. by the portion of the withdrawn spent alpha liquor. The'colnbustible content of the concentrate is burned in an oxidizing atmosphere, preferably in a combined boiler and smelting furnace. and the inorganic smelt from the furnace is dissolved as usual.-the smelt liquor containing sodium sulphate, a smaller amount of sodium carbonate, and a trace of sodium sulphide in solution. If itis desired to digest the sulphite pulp only in caustic soda sollition, a sufficient quantity of lime to causticize both the sodi/um sulphate and the sodium carbonate is'/ added to the portion of the smelt liquor available for the alpha digesting tanks. To such recovered sodium constituent and the recirculated portion of the spent alpha liquor .is added the quantity of digesting constituent in the form of caustic soda or soda ash necessary to make up the losses. in both digesting cycles. The other portion of the smelt liquor. inasmuch as it contains only sodium sulphate and sodium carbonate. is delivered directly to the acid making system. where it is treated with sulphur dioxide so as to produce the usual sodium-sulphate-sulphite liquor. I do not herein claim the subject matter of application, Serial No. 72.522. filed December 1, 1925, by Milton O. Schur and myself,

v'which discloses the digestion 'of raw cellulosic material in acid sodium base sulphite liquor to produce pulp, followed by the redigestion of such pulp in alkaline sodium base liquor.; nor do I herein claim the subject matter of my application, Serial No. 26,361, filed April 28, 1925, which discloses the redigestion in sodium base alkaline. liquor-of pulp produced in the'usual or sodium base acid sulphite liquor.

The hereinbefore described process possesses all the advantages incident to the companion application7 and offers the additional very valuable advantage in that it obviates the necessity of special recovery apparatus in a plant wherein sulphite pulp is produced from a liquor containing a sodium base digesting agent.

What l claim is:

1. A process for the production of high alpha-cellulose pulp, which comprises digesting cellulosic material iu a sulphurous acid solution of sodium compounds for the production of sulphite pulp, Washing said sulphite pulp, digesting said Washed sulphite pulp in an alkaline liquor containing sodium compounds, thereby producing a high alphacellulose pulp; Washing said high alpha-cellulose pulp,l re-employing one portion of said second-mentioned Wash Water in another digestion of sulphite pulp, mixing the other portion with said first-mentioned Wash Water and recovering the sodium compounds from the mixed Wash Waters. f

2. A process for the production of. high alpha-cellulose pulp, Which comprises digesting cellulosic material in a sulphurous acid solution of sodium compounds for the production of sulphite pulp, neutralizing the spent sulphite liquor with an alkaline sodium compound, Washing said sulphite pulp, digesting said Washed sulphite pulp in a sodium alkaline liquor, thereby producing a high alpha-cellulose pulp; Washing said high alpha-cellulose pulp, re-employing one portion of said sec d-mentionedr Wash. Water in another digestion of sulphite pulp, mixing the other portion With said firstmentioned Wash Water an recovering the sodium compounds from the mixed Wash Waters.

3. A process for the production of high alpha-cellulose fiber, which comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, re-employing one portion of the spent liquor in another digestion of like predigested pulp, mixing the other portion With spent sulphite liquor containing sodium base compounds, recovering the sodium constituent of the mixed liquors, adding one part of the recovered sodium constituents to the re-employed portion of spent liquor, and treating the second part-so as to produce a sulphite cooking liquor.

' Leeaeoa 4. A process which comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, re-employing one portion of the spent liquor in another digestion of like pulp, and mixing the other portion containing the amount-of spent sodium constituent formed in a preceding digestion With spent sulphite liquor containingsodium base compounds, recovering the sodium constituent of the mixed liquors, adding one part of the recovered sodium constituent necessary7 to effect another digestion of like pulp to the re-employed portion of spent liquor, and treating the other part of the recovered sodium constituent so as to produce a sulphite cooking liquor.

5. A process which comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, re-employing substantially all the spent liquor of one digestion in another digestion of like pulp after the addition thereto of sufficient alkaline digesting agent to make up losses therein for effective digestion,'continuing such reemployment until a predetermined sodium concentration results therein, and thereafter re-employing one portion of the hot spent liquor in another digestion, mixing the other portion containing an amount of sodium equivalent-to that Which Was formed in a previous digestion with a spent sulphite liquor containing sodiumbase compounds, re-

covering the sodium constituent of the mixed liquors`, addingA one part of the recovered sodium constituents to the re-employeol portion of spent liquor, and treating the second part so as to' produce a sulphite cookmg liquor.

6. A process of producing high alphacellulose pulp from sulphite pulp, which comprises digesting such pulp in an alkaline liquor containing alkaline digesting constituent recovered from the spenty liquor result-ing from the digestion of cellulosic Amaterial in the production of sulphite pulp, together with the spent liquor resulting from previous digestion of sulphite pulp with alkaline 1i uor.

An inter-related process of producing high alpha-cellulose pulp and sulphite pulp, which comprises digest-ing sulphite pulp in an alkaline liquor containingsodiumcompounds, digesting raw cellulosic material in a sulphite liquor containing sodium base compounds for the production of sulphite pulp, recovering the sodium constltuents from the spent liquor from said last-mentioned digestion, and making up the losses of sodium compounds therein by mixing a portion of the lspent liquor resulting from the digestion of the sulphite pulp with said spent sulphite liquor. r'

8. An inter-related process of producing' high alpha-cellulose pulp and sulphite'pulp,

in an alkaline liquor containing sodium compounds, digesting raw cellulosic material under heat ina sulphite liquor containing sodium base compounds, mixing spent liquor from said first-mentioned digestion wit-h the spent liquor from said second-mentioned digestion, and recovering the spent sodium constituent of the mixed liquors.

9. A continuous cyclic process which comprises continuously feeding and digesting predigested pulp in an alkaline liquor containing sodium compounds, washing the resulting pulp and the entrained products of the reaction, circulating a art of the wash water back for further digestion of like pulp, mixing the other part with spent sulphite liquor containing sodium base compounds, burning and smelting the inorganic content thereof in a reducing atmosphere so as to give a smelt consisting essentially of sodium carbonate and sodiumsulphide, recovering the molten smelt in water, mixing one portion of the resulting liquor with the recirculated wash water, carbonating the remaining portion whereby the sodium sulphide contained therein is converted into sodium carbonate with the evolution of hydrogen sulphide, treating said carbonated liquor with sulphur dioxide to form a sulphite liquor, and making up sodium losses in both digesting cycles by the addition of a sodium alkaline digesting agent to said recirculated wash water and recovered sodium constituent mixed therewith.

10. A continuous cyclic process which comprises continuously feeding and digesting predigested pulp in an alkaline liquor containing'sodium compounds, washing the resulting pulp and the entrained products of the reaction, circulating a part of the wash water back for further digestion of like pulp, mixing the other part with spent sulphite liquor containing sodium base compounds, burning and smelting the inorganic content thereof in a reducing atmosphere so as to give a smelt consisting essentially of sodium carbonate and sodium sulphide, recovering the molten smelt in water, mixing one portion of the resulting,A liquor with the recirculated wash water, carbonating the remaining portion whereby the sodium sulphide contained4 therein is converted int-o Sodium carbonate with the evolution of hydrogen sulphide, treating said carbonated liquor With sulphur dioxide to form a sulhite liquor, and making up sodium losses 1n both digesting cycles by the addition of a sodium alkaline digesting agent to said recirculated wash water and recovered sodium constituent mixed therewih.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

